In the remainder of the description, “container provided with a neck” is understood to be any hollow body having a neck, such as preforms, from which containers are manufactured by blow moulding or stretch blow moulding, intermediate containers in multiple blow moulding or stretch blow moulding processes, containers formed into their definitive shape and having to undergo an additional process and finished containers.
In the particular case of transporting preforms in order to heat said preforms prior to the blow moulding step, it is necessary to provide a transfer device allowing a preform to be picked up by its neck (“donning”) and to be displaced into a heating zone, generally into a tunnel oven, in which its body has to be heated to the glass transition temperature of the thermoplastic material, whilst remaining at a temperature lower than the crystallization temperature.
To carry out this transfer, transport elements are provided equipped with gripping devices formed by a mandrel which is capable of being engaged inside the neck of the preform (internal donning) and which holds said neck of the preform by pressing against the internal face of the neck.
A device for the internal donning of preforms is presented in the document FR 2 706 876 in the name of the applicant, using a gripping head having one end with a split ring. More specifically, a mandrel is provided which comprises a mandrel body with an annular groove inside of which an elastically deformable split ring is accommodated, freely rotating in the radial direction. The mandrel is introduced inside the neck of the preform until the edge of the orifice of the neck abuts against an annular shoulder of the body of the mandrel. According to this embodiment, the external diameter of the undeformed ring is substantially greater than the external diameter of the body of the mandrel and the internal diameter of the neck.
According to the same principle of internal donning, it is known according to the document FR 2 794 109 in the name of the applicant, to implement a system for conveying preforms formed by a set of transport elements each comprising a gripping device provided at its free end with a split annular ring formed by a plurality of sections; when the ring is engaged internally in the neck of a preform, the sections urged radially towards the outside by the elastic means are capable of bearing against the internal cylindrical surface of the neck.
By overdimensioning the external diameter of the split ring in the free state relative to the internal diameter of the neck of the preform which is slightly less than said external diameter of the split ring, the elastic split ring or section ring is capable of being radially deformed towards the inside during the insertion of the gripping head in the neck, whilst permitting the creation of frictional forces between the internal wall of the neck and the external periphery of the split ring or section ring, thus causing the attachment and holding of the preform on the gripping device.
The drawback of the gripping devices according to the prior art lies in the fact that they have a relatively reduced tolerance with regard to variations in the internal diameter of the necks, for example due to different ways of manufacturing preforms (different manufacturers, different manufacturing periods, etc.). Thus it is necessary to replace said gripping devices if it is desired to transport containers with very slightly different internal diameters. Due to the considerable number of gripping devices (several hundred) being able to equip a transport installation, this results in the installations in which said gripping devices are used being immobile for long periods and a considerable cost of materials kept in reserve.
In practice there is, therefore, a constant demand for a gripping device by internal donning of the container having a greater tolerance to variations in the internal diameters of the necks of the containers.
Moreover, the gripping devices by internal donning according to the prior art do not have sufficient surface contact, resulting in a risk that the container is gripped in the inclined position, and even a risk of the container being detached during the transportation thereof.
Finally, the ring sections do not have a planar external surface but a channel defining two annular projections axially separated from one another (the straight section of the sections has a very open V shape) by means of which contact with the smooth internal face of the necks is made. These ring sections formed in this manner have to be machined with a very high degree of precision in order to ensure that the two annular projections bear effectively against the neck over their entire periphery, which guarantees correct axial positioning of the container. As a result, the ring sections formed in this manner are costly.
A device for gripping the container which makes it possible to guarantee more accurately the axial alignment of the container during its transport is, therefore, repeatedly required in practice, and by having recourse to means which are less costly than those currently used.